In hybrid working machines, the hydraulic pressure supplied to hydraulic actuators (hydraulic cylinders and hydraulic motors) for driving working elements is often generated by a hydraulic pump whose drive source is an engine. In this case, the outputs of hydraulic actuators are determined by the output of the engine.
In common hybrid working machines, the driving of the engine is controlled so that the rotation speed of the engine is always constant. For example, in hybrid hydraulic shovels, the engine torque is low during their operations in a low load mode (a low load state) at the time of driving and swinging an arm, a bucket, etc. Accordingly, in a low load mode, the engine torque is increased to generate an excess torque while the engine rotation speed is kept constant. Electric power is generated by driving a generator with this excess torque, and a battery is charged with the generated electric power. Meanwhile, in a high load mode (a high load state) where a drive torque required by a hydraulic pump is higher than the rated output of the engine, the engine torque is increased while the engine rotation speed is kept constant, and an electric motor is driven with electric power from the battery to add the output of the electric motor to the output of the engine (assisting), thereby obtaining the required drive torque.
When the engine output is caused to vary while the engine rotation speed is kept constant as described above, the engine efficiency varies with the variation of the engine output, so that the fuel consumption rate of the engine also varies. Accordingly, when the engine output is caused to vary while the engine rotation speed is kept constant, the engine is not always operated at a rotation speed that results in a good fuel consumption rate.
Therefore, a hybrid working machine has been proposed that is capable of improving the fuel consumption rate at both a low load time and a high load time by efficiently driving an engine by causing the engine rotation speed to be variable based on the loading condition of the engine.